Several genome-wide association studies and case–control studies haveassociated the single nucleotide polymorphism (SNP) rs1006737, situated in CACNA1C encoding the alpha 1C subunit of the L-typevoltage-gated calcium channel, with bipolar disorder and other psychiatricdisorders. However, the causal pathway linking genetic variants in CACNA1C with increased risk for developing braindisorders remains unclear. Here, we explored the association between thers1006737 SNP and cerebrospinal fluid (CSF) markers. We found a significantassociation between the risk allele in rs1006737 and a decreased CSFhyperphosphorylated tau/total tau ratio in patients with bipolar disorder,thus linking variation in the CACNA1C gene to aneurochemical marker of neuroaxonal plasticity in those with thisdisorder.